This invention relates to a phase change composition for thermal energy storage. More particularly, the invention relates to a water/salt phase change composition including a nucleating agent to reduce supercooling of the composition.
Phase change materials are known which may be repeatedly converted between solid and liquid phases and utilize their latent heats of fusion to absorb, store and release energy to heat or cool during such phase conversions. These latent heats of fusion are greater than the sensible heat capacities of the materials. For example, in phase change materials, the amount of energy absorbed upon melting or released upon freezing is much greater than the amount of energy absorbed or released upon increasing or decreasing the temperature of the material over an increment of 10xc2x0 C.
Upon melting and freezing, per unit weight, a phase change material (PCM) absorbs or releases substantially more energy than a sensible heat storage material that is heated or cooled over generally the same temperature range. In contrast to a sensible heat storage material that absorbs and releases energy essentially uniformly over a broad temperature range, a phase change material absorbs and releases a large quantity in the vicinity of its melting/freezing point.
Phase change materials capable of storing and releasing useful amounts of energy at low temperatures have found many applications including beverage and food containers, medical wraps, and containers used to transport and store temperature-sensitive materials such as vaccines and other medicines. For example, a refrigeration system utilizing a phase change material is described in U.S. Pat. No. 5,950,450.
It is also known to use water/salt solutions as phase change materials. Eutectic water/salt solutions are useful in providing a low freezing temperature and high thermal energy storage. However, a limitation of water/salt phase change compositions is that they exhibit pronounced supercooling, i.e., cooling below their normal freezing point. In some instances, supercooling reduces the freezing temperature to about xe2x88x9231xc2x0 C., which is outside the operating range of commercial refrigerators or freezers. Thus, special freezers, or a very low freezing material such as dry ice, must be used to freeze the material.
Nucleating agents such as barium and strontium salts have been proposed for use in salt hydrate phase change materials for the purpose of reducing supercooling. See, for example, U.S. Pat. No. 4,690,769 and U.S. Pat. No. 4,613,444.
However, there is still a need in the art for a phase change composition which provides a low freezing temperature and high thermal energy storage without the problem of supercooling.
The present invention meets that need by providing a water/salt phase change composition which includes a nucleating agent to prevent supercooling of the composition. The resulting composition melts at a temperature of about xe2x88x9220xc2x0 C. and freezes at a temperature of about xe2x88x9225xc2x0 C., and may be used in a number of thermal energy storage applications.
According to one aspect of the present invention, a phase change composition is provided comprising a eutectic mixture of water and sodium chloride, and a nucleating agent in an amount of from about 0.1 to 5 weight percent. Preferably, the phase change composition includes about 1% by weight of the nucleating agent. The nucleating agent functions to reduce supercooling of the phase change composition, and is selected from the group consisting of calcium oxide, calcium hydroxide and pentaerythritol. With the addition of the nucleating agent, the phase change composition melts and freezes at a temperature between about xe2x88x9220xc2x0 C. and about xe2x88x9225xc2x0 C.
In a preferred embodiment of the invention, the eutectic mixture comprises about 72.7 weight percent water and 27.3 weight percent sodium chloride.
However, the mixture of water and sodium chloride does not have to be a eutectic mixture. Other mixtures of water and sodium chloride may be used in combination with a nucleating agent to provide a phase change composition having a freezing temperature of between xe2x88x920.58 and xe2x88x9220xc2x0 C. The desired freezing temperature may be varied by varying the amount of NaCl included in the mixture.
In another embodiment of the invention, the phase change composition further includes from about 10 to 20% by weight hydrophilic silica particles such that the phase change composition is in the form of a gel.
In yet another embodiment of the invention, the phase change composition further includes from about 25 to 35% by weight hydrophilic silica particles such that the phase change composition is in the form of a dry powder.
In another embodiment of the invention, the phase change composition may further include a water soluble thickening agent selected from the group consisting of polyvinyl alcohol, polyacrylic acid, polymethacrylic acid, polyethylene oxide, vinyl acetate/maleic anhydride copolymers, and gelatin.
The phase change composition of the present invention may be imbibed into porous substrates such as open cell foams, gypsum board, concrete, cellulose porous structures including paper, particle board, and cellulosic ceiling tiles, cotton batting, fibers and fabrics. In this embodiment, the phase change composition preferably includes a water soluble thickening agent.
The phase change composition may also be disposed in a shipping or storage container, such as a container comprising insulated panels. Preferably, the insulated panels are vacuum panels and have an R value per inch of at least 25.
The present invention also provides a method of making a phase change composition which comprises providing a mixture of water and sodium chloride; and adding a nucleating agent in an amount of from about 0.1 to 5 weight percent. The method may also include the addition of hydrophilic silica particles to form a gel or dry powder, or the addition of a water soluble thickening agent for embodiments in which the phase change composition is imbibed into a porous substrate.
Accordingly, it is a feature of the present invention to provide a phase change composition including a nucleating agent which reduces supercooling and allows the phase change composition to be used in a number of thermal storage applications including shipping or storage containers. It is also a feature of the present invention to provide a method of making such a phase change composition. Other features and advantages of the invention will be apparent from the following description and the appended claims.
The phase change composition of the present invention comprises a water/sodium chloride eutectic mixture which can store and release useful amounts of energy at temperatures of about xe2x88x9221xc2x0 C. The eutectic water/salt mixture provides a low freezing temperature and high thermal energy storage. While a eutectic mixture of 72.7% water/27.3% salt is preferred, it should be appreciated that compositions containing less than 27.3% salt may also provide useful thermal energy storage at temperatures between the xe2x88x9221xc2x0 C. melt/freeze temperature of the eutectic mixture and that of water (0xc2x0 C.).
Heretofore, a limitation of water/sodium chloride phase change compositions is that they exhibited pronounced supercooling of up to 12xc2x0 C., which reduces the freezing temperature to about xe2x88x9231xc2x0 C. With the addition of a nucleating agent, the supercooling is reduced such that the phase change composition melts and freezes at a temperature between about xe2x88x9220xc2x0 C. and xe2x88x9225xc2x0 C.
The phase change composition of the present invention may also incorporate silica particles in order to provide the phase change composition in the form of a gel or dry powder. Preferred silica particles for use in the present invention include fumed or precipitated hydrophilic silicas having a particle size of about 0.005 to 0.025 microns and a surface area of 100 m2 or more. An example is ABS silica from PPG Industries of Pittsburgh, Pa., which is a normal, hydrophilic silica having a surface area of 150 m2/gram and a particle size of about 0.022 microns. Other suitable silicas include Cabot EH-5 silica available from Cabot Corporation, which has a surface area of 350 m2/gram, and LS-500 available from DeGussa, which has a surface area of 500 m2. Also suitable is BXS-318 AND BXS 384, commercially available from PPG.
Where it is desirable to use the phase change composition in the form of a gel, the silica is preferably included in the phase change composition in an amount of from about 10 to 20% by weight. Where it is desirable to use the phase change composition in the form of a powder, the silica is preferably added to the phase change composition in an amount of from about 25 to 35% by weight.
The resulting phase change composition may be contained in an insulated shipping container for use in transporting temperature sensitive products. For example, the phase change material may be incorporated in an insulating vacuum panel such as that described in U.S. Pat. Nos. 5,943,876 and 5,950,450, both of which are incorporated herein by reference.
The phase change composition of the present invention may also be imbibed into porous substrates such as open cell foams, gypsum board, concrete, cellulose porous structures including paper, particle board, and cellulosic ceiling tiles, cotton batting, fibers and fabrics. In this embodiment, the phase change composition preferably includes a water soluble thickening agent selected from the group consisting of polyvinyl alcohol, polyacrylic acid, polymethacrylic acid, polyethylene oxide, vinyl acetate/maleic anhydride copolymers, and gelatin.